Installation and method for coating objects

ABSTRACT

An installation for coating objects, in particular vehicle wheels, has a dipping trough which is filled with a coating liquid into which the objects can be dipped. In order to reduce the complexity of the piping, there is provided a combined recirculation and filtration circuit for recirculating and filtering the coating liquid, wherein the recirculation and filtration circuit includes a pump, at least one recirculation outlet, arranged in the dipping trough, for the coating liquid, and a filtration device having a residue connection and a filtrate connection, wherein the residue connection is connected to the at least one recirculation outlet in the dipping trough.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to an installation for coating, in particular for electrodeposition painting, of objects, in particular vehicle wheels, having

a) a dip tank which is filled with a coating fluid in which the objects can be dipped.

The invention further relates to an associated method for coating objects.

2. Description of the Prior Art

Dip coating, in particular electrodeposition painting, is a method widely used in the automotive industry for coating objects, in particular for painting vehicle bodyworks and components thereof, such as, for example, the vehicle wheels.

During the electrodeposition painting operation, the objects are immersed in a dip tank in a conductive, largely aqueous coating fluid and, by applying a voltage field between the immersed objects and electrodes which are arranged in the dip tank, the coating process is started. The deposition of the paint on the objects is carried out in this instance substantially via an electrochemical conversion of binding agents and paint pigments as a result of the current flow between the electrodes and the objects. Depending on the polarity, a distinction is made between anode electrodeposition (AED) and cathode electrodeposition (CED).

During this electrochemical coating process, the composition of the coating fluid changes locally on the objects, for example, as a result of the discharge of the color pigments which bond to the coated objects from the coating fluid. In order to nonetheless achieve a uniform and predictable coating result, it is therefore necessary and known from the prior art to substantially continuously circulate the coating fluid in the dip tank using a circulation system. The physical and chemical parameters of the coating fluid in the entire dip tank are thereby kept in a homogeneous state.

By means of additional circuits, the parameters of the coating fluid are then generally adjusted. Thus, it is, for example, known to provide a filtration circuit with which a small part-flow of the coating fluid is continuously purged of solid components, such as, for example, the paint pigments. This so-called retentate is added to the coating fluid again after a corresponding preparation. In contrast, the filtrate, for example, after adjusting the pH value, is used to clean the painted objects.

It is further known to provide a heat exchange circuit by means of which the heat which is introduced into the coating fluid by the electrical power of the electro-deposition painting can be discharged again.

The disadvantage of the previously known solutions is that, for the different circuits, there have to be provided complex pipework systems, which have to be freely accessible for maintenance purposes, whereby the complexity of the pipework is again increased.

STATEMENT OF INVENTION

An object of the present invention is therefore to provide a simplified installation for coating in a dip tank.

This object is achieved according to the invention by an installation set out in the introduction, in which:

b) a combined circulation and filtration circuit is provided for circulating and filtering the coating fluid, wherein the circulation and filtration circuit comprises

a pump having a pump inlet and a pump outlet, wherein the pump inlet is at least indirectly connected to the dip tank,

at least one circulation outlet which is arranged in the dip tank for the coating fluid, and

a filtration device whose inlet is connected to the pump outlet of the pump and which has a retentate connection and a filtrate connection, wherein the retentate connection is connected to the at least one circulation outlet in the dip tank.

The inventor has recognized that, in particular with installations for coating smaller objects, such as, for example, vehicle wheels, it is not necessary to provide separate flow circuits for the filtration and for the circulation of the coating fluid. This is because, with a sufficiently large sizing of the filter device, the deposited retentate can also take over the function of circulating the coating fluid in the dip tank if it is supplied via circulation outlets into the dip tank. In this manner, the retentate which is supplied back produces the necessary bath movements of the coating fluid in the dip tank. Such circulation outlets are known per se and preferably have injector nozzles by means of which in particular in the bottom region of the dip tank there are produced occurrences of turbulence which swirl around deposited solid components of the coating fluid. Since according to the invention, therefore, only one combined circulation and filtration circuit is required instead of a plurality of separate circuits or part-circuits, the installation is on the whole simplified.

There is preferably provided an overflow tank into which the coating fluid from the dip tank can overflow, wherein the pump inlet of the pump is then connected to the overflow tank. In this manner, the coating fluid is indirectly drawn from the dip tank, wherein as a result of an overflow which is produced in this instance contaminations of the coating fluid accumulate in the overflow tank.

The pump of the circulation and filtration circuit is preferably constructed as a submersible pump and arranged in the overflow tank. This has the advantage compared with the prior art that, with regard to the sealing of the pump, a situation involving lesser requirements or even no requirements at all has to be complied with. This is because, conventionally, with normal pumps of the circulation circuit, as a result of the aggressive character of the coating fluid only labyrinth type shaft seals can be used so that coating fluid which is discharged at the seals of the pumps has to be discharged in an additional separate circuit. A submersible pump acting as the pump of the circulation and filtration circuit consequently again substantially simplifies the pipework of the installation.

In order to achieve a sufficiently high circulation flow in the coating fluid, the retentate connection of the filter device and the circulation outlet in the dip tank are preferably connected to each other without a buffer store being interposed. The combined circulation and filtration circuit can thus be operated with only one pump.

Furthermore, the filtration device may comprise an ultra-filtration module, whereby a sufficiently cleaned filtrate is discharged.

The circulation and filtration circuit may comprise a temperature control device, in particular a heat exchanger, which is preferably arranged upstream of the filtration device and by means of which heat can be removed from or supplied to the coating fluid. In this manner, it is also possible to dispense with a separate heat exchange circuit for the coating fluid, as conventionally used in the prior art. Alternatively, heat can be removed from or supplied to the coating fluid directly by means of a temperature control device which is arranged in the dip tank, for example, a cooling coil.

Furthermore, there may be provided an additional counter-container circuit which comprises a pump without any seals, in particular a diaphragm pump, by means of which the coating fluid, in particular for maintenance operations on the dip tank, can be conveyed to a counter-container and back again.

It is further advantageous for the filtrate connection of the filtration device to be at least indirectly connected to a cleaning tank, in which the objects are cleaned after the coating operation. In this manner, the filtrate, without additional pumps being necessary, can be used to clean the objects.

In this instance, there may be interposed between the filtrate connection and the cleaning tank a buffer store in which filtrate which accumulates at the filtrate connection is retained for cleaning purposes, in particular for manual cleaning purposes of the installation.

Finally, in particular with cathode electrodeposition painting, a dialysis circuit which is independent of the combined circulation and filtration circuit may be provided. This enables the acid content and the conductivity of the coating fluid to be adjusted by means of the regulation of the throughput through the dialysis circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below with reference to two embodiments and with reference to the appended drawings, in which:

FIG. 1 is a schematic illustration of an installation for dip coating vehicle wheels;

FIG. 2 is a schematic illustration of a modified installation having a buffer container for cleaning water.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an installation which is generally designated 10 for cataphoretic dip coating of vehicle wheels 12.

The installation 10 comprises a dip tank 14 which is filled with an electrodeposition paint 16 as a coating fluid. The vehicle wheels 12 which are intended to be painted are lowered into the dip tank 14 using a conveying device 18 which is only partially shown and thereby partially or completely immersed in the electrodeposition paint 16 which is substantially composed of deionized water, binding agent and paint pigments and other known additives. By applying a voltage between the vehicle wheels 12 and electrodes 20 which are also arranged in the dip tank 14, the coating process is started.

In addition to the dip tank 14, there is provided in this instance in the form of a channel which extends along the dip tank 14 an overflow tank 22 into which excess electrodeposition paint 16, as indicated by the flow arrow 24, can overflow. In this instance, conversion products of the electrodeposition paint 16 which occur during the coating process and other impurities are carried along, whereby a crest of foam 26 is formed in the overflow tank 22 above the electrodeposition paint 16.

In order to prevent solid components of the electrodeposition paint 16, for example, the paint pigments, from being deposited in the dip tank 14 and agglomerating in this case and in order to ensure that the physical and chemical parameters of the electrodeposition paint 16 are distributed in the dip tank 14 in a homogeneous manner, a circulation flow 32 is produced in the dip tank 14 using a combined circulation and filtration circuit 30.

The circulation and filtration circuit 30 comprises to this end firstly a pump which is constructed in this instance as a submersible pump 34 which is arranged in the overflow tank 22 and which has an inlet 35. From the outlet 37 thereof a line 36 leads to the primary side of a heat exchanger 38 whose secondary side is connected to a cooling system which is not shown by means of lines 40, 42.

In the continuation of the flow path of the electrodeposition paint 16, a line 44 leads from the primary side of the heat exchanger 38 to a filter device 46, more specifically to a pre-filter 48 of the filter device 46. From this pre-filter 48, another line 50 leads to an ultra-filtration module 52 of the filter device 46 which has a retentate connection 54 and a filtrate connection 56 as outlets of the filter device 46.

The retentate connection 54 of the filter device 46 at which a retentate which is enriched with solid components is discharged is connected by means of a line 58 to one or more circulation outlets 60 which are arranged in the dip tank 14 and which are each provided with an injector nozzle 62. The circulation outlets 60 are in this instance arranged and sized in such a manner that, with corresponding sizing of the retentate throughput (which is substantially controlled by means of the conveying power of the submersible pump 34), an adequate circulation flow 32 is produced in the dip tank 14 in order to prevent settlement of solid components and to achieve homogenization of the electrodeposition paint 16.

However, the filtrate connection 56 of the filter device 46, at which the aqueous components of the electrodeposition paint 16 are substantially discharged as filtrate, is connected to a cleaning tank 66 by means of a line 64. In this cleaning tank 66, the vehicle wheels 12 are cleaned after the coating process in order, for example, to remove non-adhesive, excess paint. The cleaning water 68 which is then partially enriched again with paint pigments (contaminated filtrate) is circulated in the cleaning tank 66 via an additional submersible pump 70 which is arranged in the cleaning tank 66 and associated circulating outlets 60. Furthermore, from an overflow outlet 72 which is arranged in the upper region of the cleaning tank 66, a line 74 leads to the overflow tank 22 of the dip tank 14 in order to return excess cleaning water 68 into the circulation and filtration circuit 30 of the dip tank 14.

In order to introduce fresh electrodeposition paint 16 with precisely defined chemical and physical parameters into the installation 10, it comprises a supply line 75 into the dip tank which is connected to a paint supply device which is not shown.

Since the acid content of the electrodeposition paint 16 increases during the coating process, a dialysis circuit 76 which cooperates with the electrodes 20 (and which is only partially illustrated) is further provided. This comprises tubular exchange diaphragms 78 which coaxially surround the mostly rod-like electrodes 20. The inner space of the exchange diaphragms 78 is then flushed with dialysis fluid during the coating process in order to reduce the acid content.

Finally, the dip tank 14 is connected by means of a counter-container circuit 80 to a counter-container 82 which can receive all the electrodeposition paint 16 from the dip tank 14 and thus enables the dip tank 14 to be completely emptied for maintenance purposes or the like without having to dispose of the electrodeposition paint 16. The counter-container circuit 80 comprises for this purpose a valve arrangement 84 and a diaphragm pump 86, wherein in a first position of the valve arrangement 84 the electrodeposition paint 16 can be conveyed by the diaphragm pump 86 from the dip tank 14 into the counter-container 82. In a second position of the valve arrangement, the diaphragm pump 86 together with circulation outlets 60 which are arranged in the counter-container 82 can be used to produce a circulation flow 88 in the counter-container 82 during the maintenance operations. In a third position of the valve arrangement 84, the same diaphragm pump 86 can be used to return the electrodeposition paint 16 into the dip tank 14 again.

FIG. 2 shows a slightly modified variant of the installation 10 in which the filtrate connection 56 of the filter device 46 is not directly connected to the cleaning tank 66. Instead, the line 64 extending from the filtrate connection 56 leads from that location to a buffer container 90 in which the filtrate is provided, for example, for manual cleaning purposes of the installation 10. Another line 92 on the bottom of the buffer container 92 then leads in a similar manner to the embodiment of FIG. 1 into the cleaning tank 66. 

What is claimed is:
 1. An installation for coating objects comprising: a) a dip tank which is filled with a coating fluid in which the objects can be dipped, wherein b) a combined circulation and filtration circuit is provided for circulating and filtering the coating fluid, wherein the circulation and filtration circuit comprises a pump having a pump inlet and a pump outlet, wherein the pump inlet is at least indirectly connected to the dip tank, at least one circulation outlet which is arranged in the dip tank for the coating fluid, and a filtration device whose inlet is connected to the pump outlet of the pump and which has a retentate connection and a filtrate connection, wherein the retentate connection is connected to the at least one circulation outlet in the dip tank.
 2. The installation according to claim 1, wherein there is provided an overflow tank into which the coating fluid from the dip tank can overflow, and in that the pump inlet of the pump is connected to the overflow tank.
 3. The installation according to claim 2, wherein the pump of the circulation and filtration circuit is constructed as a submersible pump and arranged in the overflow tank.
 4. The installation according to claim 1, wherein the retentate connection of the filter device and the at least one circulation outlet which is arranged in the dip tank are connected to each other without a buffer store being interposed.
 5. The installation according to claim 1, wherein the filtration device comprises an ultra-filtration module.
 6. The installation according to claim 1, wherein the circulation and filtration circuit comprises a temperature control device by means of which heat can be removed from or supplied to the coating fluid.
 7. The installation according to claim 1, wherein there is provided an additional counter-container circuit which comprises a pump without any seals, in particular a diaphragm pump, by means of which the coating fluid can be conveyed from the dip tank to a counter-container and back again.
 8. The installation according to claim 1, wherein the filtrate connection of the filtration device is at least indirectly connected to a cleaning tank, in which the objects are cleaned after the coating operation.
 9. The installation according to claim 8, wherein there is interposed between the filtrate connection and the cleaning tank a buffer store in which filtrate which accumulates at the filtrate connection is retained for cleaning purposes.
 10. A method for coating objects, comprising: a) immersing objects in a dip tank which is filled with a coating fluid, b) circulating and filtering the coating fluid in the dip tank by removing at least a portion of the coating fluid from the dip tank; filtering the coating fluid which has been removed from the dip tank in a filtration device, whereby a retentate accumulates at a retentate connection and a filtrate accumulates at a filtrate connection; and circulating the coating fluid which is present in the dip tank by returning the retentate from the retentate connection to at least one circulation outlet which is arranged in the dip tank.
 11. The installation according to claim 6, wherein the temperature control device is a heat exchanger.
 12. The installation according to claim 6, wherein the temperature control device is arranged upstream of the filtration device.
 13. The installation according to claim 9 wherein the cleaning purposes are manual cleaning purposes. 